Methods of forming a high germanium concentration silicon germanium alloy by epitaxial lateral overgrowth and structures formed thereby

ABSTRACT

A method of forming a high germanium concentration, low defect density silicon germanium film and its associated structures is described, comprising forming a dielectric layer on a substrate, patterning the dielectric layer to form a silicon region and at least one dielectric region, and forming a low defect silicon germanium layer on at least one dielectric region.

FIELD OF THE INVENTION

The present invention relates to the field of semiconductor processing,and more particularly to methods of processing germanium layers for usein microelectronic photo detectors and structures formed thereby.

BACKGROUND OF THE INVENTION

High speed infrared photodetectors are becoming increasingly importantfor optical communication signal processing, infrared imaging andmeasurement systems. Photodetectors can be used for the detection ofoptical communication signals, such as those used in thetelecommunication industry. A standard wavelength used in thetelecommunication industry is 1.3 microns. Pure silicon photodetectorsare limited in that they can only detect wavelengths up to about 1micron. The addition of germanium to a silicon film used to fabricate aphotodetector can increase the wavelength that can be detected (i.e.,photodetector responsivity). For example, using a silicon-germanium filmwhich incorporates more than 50 percent germanium can increase thephotodetector responsivity to 1.3 microns or more. However, addinggermanium to the silicon film often results in a strain to the siliconcrystal lattice that may causes defects, such as threading dislocations,to be formed in the silicon-germanium film. The proliferation of suchdefects can result in an increase in the defect concentration, or defectdensity, in the silicon germanium film, which can impair the performanceof a silicon germanium photodetector device.

Accordingly, there is a need for improved methods of forming a silicongermanium film and structures formed thereby which incorporate greaterthan 50 percent germanium concentration while exhibiting low defectconcentrations. The present invention provides such methods andstructures.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming that which is regarded as the present invention,the advantages of this invention can be more readily ascertained fromthe following description of the invention when read in conjunction withthe accompanying drawings in which:

FIGS. 1 a–1 d represent cross-sections of structures that may be formedwhen carrying out an embodiment of the method of the present invention.

FIGS. 2 a–2 e represent cross-sections of structures that may be formedwhen carrying out another embodiment of the method of the presentinvention.

FIGS. 3 a–3 i represent cross-sections of structures that may be formedwhen carrying out another embodiment of the method of the presentinvention.

FIG. 4 represents an embodiment of an optical system in accordance withthe methods of the present invention.

FIGS. 5 a–5 b represent cross-sections of structures that may be formedwhen carrying out another embodiment of the method of the presentinvention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In the following detailed description, reference is made to theaccompanying drawings that show, by way of illustration, specificembodiments in which the invention may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention. It is to be understood that the variousembodiments of the invention, although different, are not necessarilymutually exclusive. For example, a particular feature, structure, orcharacteristic described herein, in connection with one embodiment, maybe implemented within other embodiments without departing from thespirit and scope of the invention. In addition, it is to be understoodthat the location or arrangement of individual elements within eachdisclosed embodiment may be modified without departing from the spiritand scope of the invention. The following detailed description is,therefore, not to be taken in a limiting sense, and the scope of thepresent invention is defined only by the appended claims, appropriatelyinterpreted, along with the full range of equivalents to which theclaims are entitled. In the drawings, like numerals refer to the same orsimilar functionality throughout the several views.

Methods of forming a high germanium concentration, low defect densitysilicon germanium film and its associated structures are described. Themethods and associated structures of the present invention confine thedefect density of the silicon germanium film in a non-active devicearea, while providing for a low defect density silicon germanium film inan active area of the device. The formation of a such a low defectsilicon germanium film comprises forming a dielectric layer on asubstrate, patterning the dielectric layer to form a silicon region andat least one dielectric region, and forming a silicon germanium layer onat least one dielectric region and on the silicon region, wherein thesilicon germanium layer formed on the dielectric region has a low defectconcentration i.e., less than about 10⁶ defects per cm². The highgermanium concentration, low defect density layer may then be used toform a microelectronic device, such as a photodetector, and thus enablesthe use of a silicon germanium based microelectronic photodetector whichoperates at a wavelength of at least 1.3 microns.

In one embodiment of the method of the present invention, as illustratedby FIGS. 1 a–1 d, a substrate 100 may be a silicon on insulator (SOI)substrate. A SOI substrate typically comprises an insulator layersandwiched between two silicon layers. As shown in FIG. 1 a, thesubstrate 100 comprises a first silicon layer 102, an insulator layer104 disposed on the first silicon layer 102, and a second silicon layer106 disposed on the insulator layer 104. The insulator layer 104 may bean oxide, such as silicon dioxide.

A dielectric layer 108 may be formed on the second silicon layer 106 ofthe substrate 100 (FIG. 1 b). Those skilled in the art will appreciatethat the dielectric layer 108 may be formed using conventionaltechniques, such as plasma enhanced chemical vapor deposition (PECVD) orthermal oxidation techniques. The dielectric layer 108 may then bepatterned to form a silicon region 112 (by exposing the underlyingsecond silicon layer 106), and at least one dielectric region (shown inFIG. 1 c as a first dielectric region 110 and a second dielectric region110′). The dielectric layer 108 may be patterned using standardlithographic techniques well known in the art, which typically compriseapplying a photoresist on the dielectric layer 108, exposing thephotoresist in areas to be added (light mask) or removed (dark mask) andthen performing the appropriate etch.

A silicon germanium alloy layer may then be selectively formed over thesilicon region 112 and the dielectric region 110, 110′. Both a highdefect silicon germanium layer 114 and at least one low defect silicongermanium layer (shown in FIG. 1 d as a first low defect silicongermanium layer 116 and a second low defect silicon germanium layer116′) may form during a single formation process step, to form acomposite silicon germanium layer 118 which comprises the high defectsilicon germanium layer 114 and the low defect silicon germanium layer116, 116′. The high defect silicon germanium layer 114 that forms overthe silicon region 112 comprises a defect density in the range of atleast 10⁶ defects per centimeter squared. These defects may be in theform of threading dislocations that are formed to relieve the strain inthe film lattice (due to a mismatch between the germanium and thesilicon lattices) as it is being formed. The defects formed during theformation of the composite silicon germanium layer 118 are substantiallyconfined to the high defect silicon germanium layer 114.

The composite silicon germanium layer 118 may be formed by selectiveepitaxy. In selective epitaxy, germanium is added to the silicon to forma silicon germanium alloy, such as the composite silicon germanium layer118. The composite silicon germanium layer 118 thus formed may compriseat least 50 percent germanium, and preferably comprises greater than 50percent germanium, and may further comprise 100 percent germanium. Thecomposite silicon germanium layer 118 may be formed by utilizingconventional methods, suitable for the deposition of silicon germaniumfilms comprising greater than 50 percent germanium, such as by utilizinga chemical vapor deposition (CVD) or an epitaxial process, as are wellknown in the art. The deposition process may include such process gasesas SiH₂Cl₂ and GeH₄, or other suitable process gases. The processtemperature may be in a range from about 600 to 750 degrees Celsius, butmay vary depending on the process equipment and the particularapplication. It will be understood by those skilled in the art thatwhile a few examples of the process parameters may be included herein,the composite silicon germanium layer 118 may be formed by other methodsor processes that form a silicon germanium alloy comprising greater than50 percent germanium.

The low defect silicon germanium layer 116, 116′ that forms over thedielectric region 110, 110′ may comprise a defect density below 10⁶defects per centimeter squared, and may preferably be virtually defectfree, because the defects in the composite silicon germanium layer 118are substantially confined to the high defect silicon germanium layer114. Selective epitaxy in the method of the present invention may alsobe called epitaxial lateral overgrowth (ELO) because the low defectsilicon germanium layer 116, 116′ forms laterally over at least onedielectric region (shown in FIG. 1 d as forming laterally over the firstdielectric region 110 and the second dielectric region 110′). Thus, themethod of the present invention provides a high germanium concentration,low defect silicon germanium layer 116, 116′.

In another embodiment of the present invention (FIGS. 2 a–2 e), asubstrate 200, which may be a SOI substrate (similar to the SOIsubstrate 100 in FIG. 1 a), comprises first silicon layer 202, aninsulator layer 204 disposed on the first silicon layer 202, and asecond silicon layer 206 disposed on the insulator layer 204 (FIG. 2 a).The insulator layer 204 may be an oxide, such as silicon dioxide.

A buffer layer 207 may be formed on the second silicon layer 206 of thesubstrate 200 (FIG. 2 b). The buffer layer 207 may comprise silicongermanium, and may be formed by CVD epitaxy, or by any such methods usedto form a silicon germanium alloy, such as the composite silicongermanium layer 118 as previously discussed herein. The buffer layer 207may be formed by using a grading technique, as is well known in the art,in which the buffer layer 207 is formed by sequentially increasing thepercentage of germanium in the buffer layer 207 as the buffer layer 207forms. The thickness of the buffer layer 207 is preferably less than 1micron thick. The buffer layer 207 may also be formed by forming a thinlayer of non-graded silicon germanium (preferably less than 1 micron inthickness), on the second silicon layer 206. The buffer layer 207 maythen be cycled through an annealing process, as is known in the art.Defect concentrations of about 5×10⁶ defects per centimeter squared maybe obtained by using such a technique to form the buffer layer 207.

A dielectric layer 208 may be formed on the buffer layer 207 (FIG. 2 c).The dielectric layer 208 may then be patterned to form a buffer region212 (by exposing the underlying buffer layer 207) and at least onedielectric region (shown in FIG. 2 d as a first dielectric region 210and a second dielectric region 210′). A composite silicon germaniumlayer 218 (similar to the composite silicon germanium layer 118) maythen be formed (FIG. 2 e) on the buffer region 212 and the dielectricregion 210, 210′ by selective epitaxy, as previously described. Thecomposite silicon germanium layer 218 comprises at least 50 percentgermanium. The composite silicon germanium alloy 218 forms verticallyover the buffer region 212, and also forms laterally over the dielectricregion 210, 210′. The composite silicon germanium layer 218 furthercomprises a high defect silicon germanium layer 214 (similar to highdefect region 114) and at least one low defect silicon germanium layer(shown in FIG. 2 e as a first low defect silicon germanium layer 216 anda second low defect silicon germanium layer 216′, similar to low defectregion 116, 116′). The high defect silicon germanium layer 214 thatforms over the buffer region 212 may comprises a defect density in therange of about 5×10⁶ defects per centimeter squared or lower, but thedefect density of the high defect silicon germanium layer 214 is lessthan if the high defect silicon germanium layer 214 were grown directlyon a silicon substrate, as in high defect layer 114. The defects formedduring the formation of the composite silicon germanium layer 218 aresubstantially confined to the high defect silicon germanium layer 214.

The low defect silicon germanium layer 216, 216′ forms over thedielectric region 210, 210′ by epitaxial lateral overgrowth. The defectdensity in the low defect silicon germanium layer 216, 216′ is below10⁶defects per centimeter squared, and may be virtually defect free,because the defects in the composite silicon germanium layer 218 aresubstantially confined to the high defect silicon germanium layer 214.Thus, the method of the present invention provides a high germaniumconcentration, low defect silicon germanium layer 216, 216′ whichincorporates a buffer layer 207 which reduces the defect density of thehigh defect silicon germanium layer 214.

In another embodiment (FIG. 5 a), a thin buffer buffer layer 507(similar to buffer layer 207) may be selectively formed over a siliconregion 512 (similar to the silicon region 112). The silicon region 512and a dielectric region 510, 510′ (similar to dielectric region 210′,210′), are disposed on a substrate 500 (FIG. 5 a). The thin buffer layer507 is preferably less than 1 micron in thickness. A composite silicongermanium layer 518 (similar to the composite silicon germanium layer218) may then be formed over the silicon region 512 and the dielectricregion 510, 510′ (FIG. 5 b). The composite silicon germanium layer 518may comprises a high defect silicon germanium layer 514 (similar to thehigh defect silicon germanium, layer 214) and a low defect silicongermanium layer 516, 516′ (similar to the low defect silicon germaniumlayer 216, 216′). Thus, the current embodiment provides a high germaniumconcentration, low defect silicon germanium layer 516, 516′ whichincorporates a thin buffer layer 507 which is selectively grown over thesilicon region 512.

In another embodiment of the present invention, a method of forming amicroelectronic device, such as a light-sensing device 334 (FIG. 3 i)and structures formed thereby is illustrated in FIGS. 3 a–3 i. Asubstrate 300, which may be a SOI substrate (similar to the SOIsubstrate 100 in FIG. 1 a), comprises first silicon layer 302, aninsulator layer 304 disposed on the first silicon layer 302, and asecond silicon layer 306 disposed on the insulator layer 304 (FIG. 3 a).The insulator layer 304 may be an oxide, such as silicon dioxide. Itwill be understood by those skilled in the art that the substrate 300may include various structures, such as a trench structure, as is wellknown in the art.

A dielectric layer 308 may then be formed on the second silicon layer306 of the silicon substrate 300 (FIG. 3 b) using conventional methodsknown in the art, such as CVD. The dielectric layer 308 may then bepatterned to form a silicon region 312 and at least one dielectricregion (shown in FIG. 3 c as a first dielectric region 310, and a seconddielectric region 310′).

A composite silicon germanium layer 318 (similar to the compositesilicon germanium layer 218) may then be formed (FIG. 3 d) by selectiveepitaxy, as previously described, wherein the composite silicongermanium layer 318 comprises at least 50 percent germanium. Thecomposite silicon germanium layer 318 forms vertically over the siliconregion 312 and also forms laterally over the dielectric region 310,310′. The composite silicon germanium layer 318 further comprises a highdefect silicon germanium layer 314 and a low defect silicon germaniumlayer (shown in FIG. 3 d as a first low defect silicon germanium layer316, and a second low defect silicon germanium layer 316′). The highdefect silicon germanium layer 314 that forms over the silicon region312 comprises a defect density in the range of about 5×10⁶ defects percentimeter squared. The defects formed during the formation of thecomposite silicon germanium layer 318 are substantially confined to thehigh defect silicon germanium layer 314.

The low defect silicon germanium layer 316, 316 forms over thedielectric region 310, 310′ by epitaxial lateral overgrowth. The defectdensity in the low defect silicon germanium layer 316, 316′ is below10⁶defects per centimeter squared, and may be virtually defect free,because the defects in the composite silicon germanium layer 318 aresubstantially confined to the high defect silicon germanium layer 314.

The composite silicon germanium layer 318 may be patterned by methodsknown in the art to form a patterned composite silicon germaniumstructure 319 (FIG. 3 e). Such structures may include a waveguide, orother such optical devices used in optical communication systems, forexample. The patterned composite silicon germanium structure 319 may befurther processed to form a p-i-n photodiode 320 (for ease ofunderstanding, shown separated from the patterned composite silicongermanium structure 319 in FIG. 3 f). The p-i-n photodiode 320 maycomprise at least one p-type low defect silicon germanium layer (shownin FIG. 3 f as a first p-type low defect silicon germanium layer 321 anda second p-type low defect silicon germanium layer 321′), an intrinsiclow defect silicon germanium layer 322, and an n-type low defect silicongermanium layer 323.

The p-i-n photodiode 320 may be formed from the patterned compositesilicon germanium structure 319 by first forming a mask 324, as is wellknown in the art, on the patterned composite silicon germanium layer 319(FIG. 3 g). The mask 324 may comprise at least one implantation opening325 to the low defect silicon germanium layer 316. An ion implantation326, as is well known in the art, may be performed in which an n-typedopant (impurity) is implanted into an upper region 317 of the first lowdefect silicon germanium layer 316. The n type dopant may include suchdopants as phosphorus, arsenic, or other such n-type dopant materials asare well known in the art. In this manner, the upper region 317 of thefirst low defect silicon germanium layer 316, which normally comprisesan intrinsic level doping (i.e., there is no excess of either a p typeor n-type dopant material) is doped with n-type dopant to form the ntype low defect silicon germanium layer 323 of the p-i-n diode 320 (seeFIG. 3 h). The depth 328 of the upper region 317 (as shown in FIG. 3 f)may be adjusted according to the design parameters of the light sensingdevice 334.

The intrinsic low defect silicon germanium layer 322 of the p-i-nphotodiode 320 comprises the region of the first low defect silicongermanium layer 316 beneath the n type low defect silicon germaniumlayer 323 (which is normally intrinsic, as previously described, seeFIG. 3 h). The p-type low defect silicon germanium layer 321, 321′ isformed by first forming at least one metallization opening (illustratedin FIG. 3 h as a first metal mask opening 330, a second metal maskopening 330′ and a third metal mask opening 330″) in the mask 324. Ametal contact (shown in FIG. 3 i as a first metal contact 332, a secondmetal contact 332′ and a third metal contact 332″) may then be formed onat least one low defect silicon germanium contact region (shown in FIG.3 h as a first low defect silicon germanium contact region 327 and asecond low defect silicon germanium contact region 327′) in a mannerwell known in the art. The low defect silicon germanium contact region327, 327′ is a region of the first low defect silicon germanium layer316 that is not beneath the n type low defect silicon germanium layer323. The first metal contact 332 and the third metal contact 332″ thatform on the first low defect silicon germanium contact region 327 andthe second low defect silicon germanium contact region 327′respectively, may dope the low defect silicon germanium contact region327, 327′ with a p-type dopant. For example, where the metal contact332, 332′″ is aluminum, the aluminum metal itself may provide a p-typedopant to the low defect silicon germanium contact region 327, 327′,thus forming the p-type low defect silicon germanium layer 321, 321′ ofthe p-i-n diode 320 (FIG. 3 i). The second metal contact 332′ may alsoform on the n type low defect silicon germanium layer 323 during thesame process step which forms the first metal contact 332 and the thirdmetal contact 332″. Metallization to the photodiode 320 is thus enabledand in this manner the light sensing device 334, which form a portion ofan optical device, such as a waveguide, as is well known in the art maybe formed.

It will be appreciated by those skilled in the art that the doping ofthe p-i-n photodiode 320 of the light sensing device 334 may beperformed by using other techniques known in the art with which to dopethe n and p type layers of such photodiodes, such as the n type lowdefect silicon germanium layer 323 and the p type low defect silicongermanium layer 321, 321′. Such techniques may include (by example andnot limitation) using a CVD deposition, or other non-implant techniquesof doping a silicon germanium film as are known in the art. Furthermore,the light sensing device 334 may include other types of photodiodes,such as a metal semiconductor metal (MSM) diode, as is well known in theart (and thus will not be described further herein), and is not limitedto a p-i-n photodiode.

The light sensing device 334 formed according to the method of thecurrent embodiment may be used in any number of optical communicationsystems. For example, it is capable of being used in thetelecommunications industry, since it is capable of operating above 1.3microns, which is the telecommunications industry standard operatingwavelength. FIG. 4 is a diagram illustrating one embodiment of anoptical communication system 400 including an optical device 404, suchas the light sensing device 334 of the present invention. In variousembodiments the optical device 404 may include a light sensing device,such as a waveguide. In the depicted embodiment, the opticalcommunication system 400 includes an optical transmitter 402 to transmitan optical beam 408. An optical receiver 406 is optically coupled toreceive the optical beam 408. It is appreciated that the opticaltransmitter 402 and the optical receiver 406 may also include opticaltransceivers and therefore have both transmitting and receivingcapabilities for bi-directional communications. In one embodiment, theoptical device 404 is optically coupled between optical transmitter 402and optical receiver 406. In the illustrated embodiment, optical device404 is shown to be at the receiving end of optical communication system400. In other embodiments, the optical device 404 may be disposed atvarious locations along a transmission path or at the transmitting endof the optical communication system 400. In one embodiment, the opticaldevice 404 may be included in a waveguide and be utilized in for examplean add/drop filter, as is well known in the art, that enables theaddition or extraction of a channel from a wave division multiplexed(WDM, also well known in the art) optical beam 408 transmitted fromoptical transmitter 402 along an optical path. Thus, an optical beam 410having a specific wavelength is output from the optical device 404.

As described above, the present invention provides methods andassociated structures of forming a high germanium concentration, lowdefect density silicon germanium film in order to enable microelectronicdevices which are capable of operating at wavelengths of 1.3 microns andabove. In addition, because there are less defects per centimetersquared in the low defect silicon germanium layer of the presentinvention, the dark current of a device (a measurement of currentleakage during non-operating times), may be to about 10 microamperes orbelow. This greatly enhances the reliability and performance of a devicefabricated according to the various embodiments of the presentinvention.

Although the foregoing description has specified certain steps andmaterials that may be used in the method of the present invention, thoseskilled in the art will appreciate that many modifications andsubstitutions may be made. Accordingly, it is intended that all suchmodifications, alterations, substitutions and additions be considered tofall within the spirit and scope of the invention as defined by theappended claims. In addition, it is appreciated that the fabrication ofa multiple metal layer structure atop a substrate, such as a siliconsubstrate, to manufacture a silicon device is well known in the art.Therefore, it is appreciated that the Figures provided herein illustrateonly portions of an exemplary microelectronic device that pertains tothe practice of the present invention. Thus the present invention is notlimited to the structures described herein.

1. A method of forming a photodiode structure comprising: forming adielectric layer on a substrate; patterning the dielectric layer to forma silicon region and at least one dielectric region; and forming a lowdefect silicon germanium layer by lateral epitaxial overgrowthcontacting a portion of a top surface of the at least one dielectricregion, wherein the portion is greater than about 50%; wherein formingthe dielectric on the substrate further comprises: forming a bufferlayer on the substrate, wherein forming the buffer layer comprisesutilizing a grading technique, and forming a dielectric layer on thebuffer layer.
 2. The method of claim 1 wherein forming the low defectsilicon germanium layer comprises forming the low defect silicongermanium layer wherein the low defect silicon germanium layer comprisesgreater than 50% germanium.
 3. A method of forming a photodiodecomprising: forming a dielectric layer on a silicon substrate;patterning the dielectric layer to form a silicon region and adielectric region; forming a low defect silicon germanium layer bylateral epitaxial overgrowth contacting a portion of a top surface ofthe at least one dielectric region, wherein the portion is greater thanabout 50%; doping the low defect silicon germanium layer with an n typedopant, wherein an n type low defect silicon germanium layer is formed;and forming metal contacts on the n type low defect silicon germaniumlayer and on a low defect silicon germanium contact region whereinforming the dielectric on the substrate further comprises: forming abuffer layer on the substrate, wherein forming the buffer layercomprises utilizing a grading technique, and forming a dielectric layeron the buffer layer.
 4. The method of claim 3 wherein forming metalcontacts on the low defect silicon germanium contact region furthercomprises forming a p type low defect silicon germanium layer.
 5. Themethod of claim 3 wherein forming the low defect silicon germanium layercomprises forming the low defect silicon germanium layer wherein the lowdefect silicon germanium layer comprises less than about 10⁶ defects percm².
 6. The method of claim 3 wherein forming the low defect silicongermanium layer comprises forming the low defect silicon germanium layerwherein the low defect silicon germanium layer comprises about 50% orgreater germanium.
 7. The method of claim 3 wherein forming a dielectriclayer on a silicon substrate comprises forming the dielectric layer on asilicon on insulator (SOI) substrate.